Hybrid space system based on a constellation of low-orbit satellites working as space repeaters for improving the transmission and reception of geostationary signals

ABSTRACT

Satellite telecommunications system comprises a transmitting/receiving surface terminal associated with a user substantially on a surface of the Earth, a geostationary satellite configured to receive/transmit signals from/to a predefined coverage area with a line of sight to the geostationary satellite, and a traveling satellite moving above the surface of the Earth. The traveling satellite repeats signals received from the surface terminal towards the geostationary satellite and/or repeat signals received from the geostationary satellite towards the surface terminal. The same frequency band is used to communicate between the surface terminal and the traveling satellite and between the traveling satellite and the geostationary satellite. The tracking/telemetry and command signals of the traveling satellite are relayed by the geostationary satellite.

The invention belongs to the field of long-distance data transmissionsystems. It relates more specifically to systems and methods of datacommunication (data transfer, remote control, monitoring terminals,etc.) between users equipped with small mobile terminals.

BACKGROUND OF THE INVENTION AND PROBLEM STATEMENT

The issue of long-distance data transmission from or to a mobileterminal is of particular concern with regard to connections betweencomputers (machine to machine). This field of transmission is thereforecharacterized by a need for significantly lower data throughput ratesthan for image or Internet types of connections.

A first approach to this problem is known, followed by existing datatransmission systems such as Orbcomm and Argos, which use constellationsof low-orbit satellites (LEO, for Low Earth Orbit). In this approach,the normal mode of operation for each LEO low-orbit satellite requiresit to be simultaneously visible firstly by a ground control andconnection station and secondly by a user terminal.

The satellite thus serves as a communications link between the twoparts, and the latency between acknowledgments of receipt and messagesis a function of the distance between the satellite and the groundstation (GES, for Gateway Earth Station).

However, the coverage provided by the ground station network of systemsusing low-orbit satellites, such as Orbcomm and Argos, is limited by thedeployment of ground stations (GES) and the existing systems onlyprovide a limited coverage of the Earth in this mode. In effect, eachground station allows coverage over a radius of about 3000 km, and eachof these systems comprises some twenty ground stations.

It can therefore easily be seen that the coverage areas have large“blind” areas where the system cannot be used. In particular, theseareas cover a large portion of the oceanic areas, even a significantportion of continental areas such as Africa and Australia.

In cases where the LEO satellite has no simultaneous visibility of boththe user terminal and the GES ground control station, a type ofcommunications method that stores then sends (known to the personskilled in the art as Store & Forward) must be used. In this method themessage is stored on board the satellite, which continues moving in itsorbit until it flies over the GES ground station, to which it deliversthe stored message.

With this communications method communication delays are long and maketwo-way communications in acceptable conditions difficult, given thatthe delays are typically between several minutes and the 100 to 150minutes duration of a complete orbit of the LEO satellite.

Hybrid telecommunications systems for transmitting data between usersare also known. These hybrid systems comprise geostationary satellitesand a low-orbit satellite constellation.

In particular a first patent document. FR 2764755 U.S. Pat. No.6,208,625: Method and apparatus for increasing call-handling capacityusing a multi-tier satellite network, can be cited.

This document describes a network formed of LEO and geostationary (GEO)satellites able to communicate to each other. On the ground, userterminals are capable of reception/transmission (Rx/Tx) with LEO and GEOsatellites. The LEO component filters the traffic received from theterminals and, depending on the urgent nature of the traffic received,directs this traffic either internally to the LEO or to the GEO.

A second patent document, EP 0883252/U.S. Pat. No. 6,339,707: Method andsystem for providing wideband communications to mobile users in asatellite-based network, proposes a satellite communications system thatpermits global coverage, reduced transmission delays (Tx), and maximizeduse of the system's capacity (wideband communication satellite via theinterconnection of several medium-orbit—MEO—andgeostationary—GEO—constellations).

The MEO and GEO satellites communicate directly to each other byinter-satellite links, which enables traffic routing (for voice anddata) on board the satellites according to certain rules.

In addition, this document proposes having the very high frequencyspectrum shared and re-used between the GEO and MEO satellites (e.g.between 40 and 60 GHz), in order to enable the function known under thename “seamless handover” for portable terminals (passing from a mobilenetwork to a fixed network with no interruption to the communication inprogress).

It is clear that the current hybrid systems are highly complex,synonymous with high implementation and use costs.

OBJECTIVES OF THE INVENTION

The objective of this invention is to propose a new system of datacommunications between mobile users.

A second objective of the invention is to improve the performance andreduce the cost of a system of data communications between mobiledevices.

DESCRIPTION OF THE INVENTION

To this end, the invention envisages in the first place atelecommunications system, intended for low-throughput data transferbetween at least two users located substantially on the surface of acelestial body;

the system comprises:

a plurality of transmitting/receiving surface terminals, each associatedto a user,

one or more stationary means above the surface of the celestial body,said stationary means being able to transmit data from and to apredefined coverage area with a line of sight to the stationary means,

and one or more signal repetition means for signals transmitted and/orreceived from both the stationary means and the surface terminals, saidrepetition means traveling above the surface of the celestial body; asame frequency band being used for communications between the surfaceterminals and the repetition means and for communications between therepetition means and the stationary means.

Throughout this application, “users located substantially on thesurface” means, in particular, terrestrial, maritime or aeronauticalusers. Similarly, the surface terminals are accepted to be, for example,placed in terrestrial, maritime or aeronautical means.

According to a preferred embodiment, the system comprises at least oneground connection station (GES) for the GEO stationary means.Communications between the ground and the space repetition means areperformed via the GEO stationary means and GES connection stations ofthese GEO stationary means. These communications comprise both the dataexchanges between users and potentially the remote and telemetrycommunications of the space repetition means. Thus this embodiment doesnot require the ground connection station assigned to the spacerepetition means to be used.

It is clear that, apart from the case where both users have surfacetransmitting/receiving terminals, a user of the system can also beconnected to a terrestrial network (IP, PSTN, etc.) via a GES groundconnection station.

According to a preferred implementation, at least one stationary meansis installed on board a satellite in geostationary orbit around thecelestial body.

Similarly, preferably, at least one repetition means is on board alow-orbit satellite traveling around the celestial body.

In other words, the invention envisages in particular a system forremote data communications between mobile devices, the system usingpayloads on board one or more geostationary satellites and on alow-orbit traveling satellite constellation, in which the satellitestraveling in orbit act as space repeaters for the signals transmittedand/or received from the geostationary satellites.

Using low-orbit satellites acting as repeaters/amplifiers of signals inthe same frequency band as geostationary satellites makes it possible toimprove the transmission or reception of signals from geostationarysatellites, so as to obtain the best compromise in terms of thecost/coverage ratio and services. As a result, the system makes itpossible to improve the performances of the services offered by thegeostationary satellite, potentially create new services and expand thegeostationary satellite's coverage (e.g. to polar regions).

The constellation of low-orbit traveling satellites, acting as spacerepeaters, can be, in a preferred embodiment, an LEO (Low Earth Orbit)constellation or alternatively an MEO (Medium Earth Orbit)constellation.

An improvement in the system's performance is thus obtained, compared tothe systems of the prior state of the art, due to the fact that therepeater satellite is closer to the Earth than the geostationary orbit,and consequently the losses due to the propagation of signals in freespace are much lower.

In an advantageous implementation, at least one repetition means isinstalled on board a satellite moving in a polar or quasi-polar orbit(orbital inclination greater than 70) around the celestial body.

A polar orbit makes it possible to improve coverage of high-latitudeareas, where good conditions of service cannot be provided by ageostationary satellite.

This hybrid solution (geostationary component and complementary spacecomponent) combines the advantages of each component by using the samefrequency spectrum on the LEO (or MEO) satellites, on the GEO(Geostationary Earth Orbit) satellites and on the relay link between LEOand GEO.

The frequency spectrum used for this hybrid solution can be in any ofthe bands allocated to satellite telecommunications services (from lowbands, e.g. UHF or VHF, up to high frequencies such as the Ka or Qbands).

Preferably, the frequency band used is the L band (between 0.9 and 2.0GHz), which is especially suitable for satellite mobile communications.

Indeed, a significant characteristic of the invention consists of thesame frequency band being shared between a geostationary satellite and alow-orbit satellite constellation, over the user link. In other words,the communications between the users and the LEO satellite constellationand the communications between the GEO satellite and the LEO satelliteboth use the same frequency band. This produces a significant advantagecompared to the solutions of the prior state of the art.

A secondary characteristic of the invention, valid when the terminal isin the GEO satellite's coverage area, consists of using the samefrequency band for the direct communications between the users and theGEO satellite.

In this case, the frequency band is also used for communications betweenuser terminals and at least one stationary means.

According to a first embodiment,

-   -   at least one repetition means comprises means of performing an        amplification without frequency translation of the signal        received from the stationary means.    -   the air interface used is a CDMA (“Code Division Multiple        Access”) type of interface,    -   and at least one user terminal comprises means of managing the        arrival of two signals bearing Doppler and delay differences.

In this case, preferably, the user terminal's means of managing thearrival of two signals comprising Doppler and delay differences are a“Rake” type of receiver well known to the person skilled in the art.

According to an alternative embodiment:

-   -   the air interface is a TDMA type,    -   the system uses two separate signals: one for the stationary        means and one for the repetition means,    -   time-division multiplexing is used for distributing the capacity        between the stationary means and the repetition means, with        guard intervals and Doppler pre-compensation at the repetition        means.

Advantageously, at least one user terminal comprises means of using thespace diversity or MIMO (Multiple Input Multiple Output) techniques torecombine the signals coming from both a stationary means and arepetition means.

According to another embodiment,

-   -   at least one LEO repetition means relays the signal, in a        transparent or regenerative way, without frequency translation        of the received signal    -   and the air interface comprises means of limiting interference        at a user terminal between the signals from a GEO stationary        means and the signals relayed by an LEO repetition means.

According to a second embodiment,

-   -   at least one LEO repetition means relays the signal, in a        transparent or regenerative way, in an adjacent channel, before        its retransmission,    -   and the communications system comprises a coordination entity        for coordinating the frequency plans between the GEO stationary        means and the LEO repetition means.

In a second aspect the invention envisages a telecommunications method,intended for low-throughput data transfer between two users locatedsubstantially on the surface of a celestial body; the first user beingequipped with a user terminal, with a line of sight to at least onerepetition means, and the repetition means having a line of sight to atleast one stationary means, the method using a telecommunications systemas described,

the method comprises in particular steps in which:

-   -   the first users terminal transmits a first uplink signal,        representative of data to be transmitted, to the repetition        means,    -   the repetition means receives and amplifies the first signal        transmitted by the user terminal on the ground and transmits it        in the form of a second uplink signal to the stationary means in        the same frequency band, the stationary means ensures the good        final transmission of data to the second user.

The invention also envisages a telecommunications method, intended forlow-throughput data transfer between two users located substantially onthe surface of a celestial body; the second user being equipped with asecond user terminal, with a line of sight to at least one repetitionmeans, and the repetition means having a line of sight to at least onestationary means, the method using a telecommunications system asdescribed,

the method comprises in particular steps in which:

-   -   the stationary means retransmits a signal, representative of        data to be transmitted, received from a first user, to the        repetition means in the form of a first downlink signal,    -   the repetition means receives and amplifies the first downlink        signal transmitted by the stationary means and transmits it to        the second user's terminal on the ground in the same frequency        band in the form of a second downlink signal,

It is understood that the two parts of the method can be used at thesame time.

Preferably, the re-use of frequencies between the different componentsof the communications system is performed in a coordinated way tominimize intra-system interference.

In effect the two constellations, the one in low orbit and the other ingeostationary orbit, here use as far as possible the same availablespectrum, preferably in the L band (between 0.9 and 2.0 GHz) but all thebands statutorily allocated to satellite telecommunications services,from low bands, e.g. UHF or VHF, up to high frequencies such as the Kaor Q bands, can be envisaged.

In other aspects the invention envisages a repetition means and a userterminal for a communications system as described.

As the preferential or particular characteristics and the advantages ofthis repetition means and of this user terminal are similar to those ofthe system as described in brief above, these advantages are notrepeated here.

BRIEF DESCRIPTION OF THE FIGURES

The goals and advantages of the invention will be better understood inreading the description and drawings of a particular embodiment, givenas a non-limiting example, for which the drawings show:

FIG. 1: the general architecture of the system;

FIG. 2: an illustration of the positions of the LEO and GEO satelliteson a planisphere, at a given time;

FIG. 3: a table of the orders of magnitude for the delays between a GEOsatellite's signals and an LEO satellite's signals for different LEOorbit altitudes;

FIG. 4: the coverage areas of the LEO and GEO satellites of theconstellation described.

DETAILED DESCRIPTION OF AN EMBODIMENT OF THE INVENTION

The system architecture is shown in FIGS. 1 and 2. As can be seen inthese figures, the proposed system uses two satellite constellations.The first constellation comprises one or more geostationary satellites(also called GEO in the rest of the description).

In this case the system, described here in a non-limiting example, isbased on a constellation of three geostationary satellites, GEO1, GEO2,GEO3, placed in geostationary orbit over three main continental areas(e.g. at longitudes 265° E, 25° E, 145° E respectively, as shown in FIG.2). The geostationary satellites GEO1, GEO2, GEO3 operate in the bandknown as MSS L (1.5/1.6 GHz).

The constellation of geostationary satellites GEO1, GEO2, GEO3 iscontrolled by one or more terrestrial controls, which are placed in lineof sight of the geostationary satellites GEO1, GEO2, GEO3 they control,and which perform the control and remote control functions. Theconstellation of geostationary satellites GEO1, GEO2, GEO3 is connectedto terrestrial communications networks by one or more GES terrestrialconnection stations, placed in line of sight of the geostationarysatellites GEO1, GEO2, GEO3 via an FL (Feeder Link) link, in a way knownper se.

The system is completed by a second constellation of three satellitestraveling in low or medium orbit (satellites known as LEO or MEG), withorbit altitudes typically between 400 and 20000 km, which act as spacerepeaters. It is clear that the system can use a larger or smallernumber of satellites in each of the LEO and geostationaryconstellations, the difference being a more or less complete coverage ofthe Earth.

In the example described here, the traveling satellites are assumed tobe of a type moving in low orbit (known as LEO), and placed inheliosynchronous orbit at an altitude of 567 km with an inclination of97.7° in three different orbital planes (with right ascension nodes at0°, 60° and 120°). It is noted that the heliosynchronous orbit isdefined by the fact that each satellite, after several orbits, passesthe line of sight of the same point on the Earth at the same local solartime.

This system uses three low-orbit satellites, LEO1, LEO2, LEO3; theirorbital courses are shown in FIG. 2 as a non-limiting example. In thisexample these three low-orbit satellites. LEO1, LEO2, LEO3, could be“piggyback” payloads installed on board satellites whose main payload isdedicated to another mission, e.g. observing the Earth.

It is clear that the constellation of traveling satellites, LEO1, LEO2,LEO3, can comprise satellites moving in orbits with different altitudesor inclinations.

These low-orbit satellites, LEO1. LEO2, LEO3, operate in the samefrequency band as the geostationary satellites, GEO1, GEO2, GEO3, and inthis case in the MSS L band (1.5/1.6 GHz).

The communications system is aimed at any user, in particular a user whois mobile on the surface of the Earth, and equipped with a transmittingreceiving terminal REC1, transmitting or receiving data to or fromanother user, who possibly has a transmitting/receiving terminal REC2,and is also possibly mobile on the surface of the Earth. User REC3 canalso be connected to a terrestrial network (IP, PSTN, etc.) and beconnected to user REC1 via the GES,

Each user terminal REC1 and REC2 is a transportable terminal, comprisingin particular a user interface, e.g. a type such as a keyboard, touchscreen or data link to an item of electronic equipment, a battery and/orpower supply means, a processor and/or electronic control unit, means ofstoring programs or data, and means of transmitting and receivingsignals, operating in the MSS L frequency band, in this exampledescribed here in a non-limiting way.

In this example each user terminal REC1 and REC2 has an omni-directionalantenna, designed to receive signals coming either from any one of thelow-orbit satellites, LEO1, LEO2, LEO3, or from any one of thegeostationary satellites, GEO1, GEO2, GEO3.

In the implementation described here, each user terminal REC1 and REC2comprises a “Rake” type of receiver, well known to the person skilled inthe art, for the forward channel. It is noted that a Rake receiver is aradio receiver, originally designed to compensate for attenuation due tothe multiple paths of radio waves for the terrestrial systems. It isbased on the concept that the reflected signals can be distinguished(typically in the case where a CDMA multiplexing technique is used) andtherefore can be combined in a suitable way, thus taking advantage ofmultiple propagations. For the return channel, the GEO satellites areassumed to be transparent and the Rake receiver is located at the GES(the GEO satellites' connection stations).

However, it should be noted that in the case where the GEO satellitesare of a regenerative type, it would be necessary to install a Rakereceiver on board them (replacing the receiver at the GES).

Operating Mode

A communication between two user terminals REC1, REC2, assumed to be inline of sight to two low-orbit satellites. LEO1, LEO2 respectively, andthe same geostationary satellite GEO1, comprises several steps, as shownin FIG. 1:

-   -   the first user terminal REC1 transmits a first signal S1 to the        first low-orbit satellite LEO1,    -   the low-orbit satellite LEO1 receives and amplifies signal S1        transmitted by user terminal REC1 on the ground and transmits it        in the form of signal S2 to the geostationary satellite GEO1,    -   the geostationary-orbit satellite GEO1 receives signal S2 and,        if conditions allow it, signal S1 and re-transmits them in the        form of signal S3 to the second low-orbit satellite 1102, either        directly (with routing on board the satellite) or via the GES        connection station. Signals S1 and S2 are processed with a Rake        receiver, either on board (in the scenario with on-board        routing) or at the GES station (this solution is preferred for        reasons of a simpler implementation).    -   the low-orbit satellite LEO2 receives and amplifies signal S3        transmitted by geostationary-orbit satellite GEO1 and transmits        it in the form of signal S4 to user terminal REC2 on the ground,    -   user terminal REC2 receives signal S4 and possibly signal S3, if        conditions allow it. A Rake receiver allows these two signals to        be recombined at the user terminal.

In a case involving user terminals in line of sight to two differentgeostationary satellites GEO1 GEO2, the link between the two userterminals comprises in addition a segment of communication between thesesatellites, for example, but not exclusively, through GES connectionstations and ground links or through a direct GEO inter-satellite link,if it exists.

It is naturally understood that carrying out a communication between auser REC1 having a portable transmitting/receiving terminal, and anotheruser REC3 connected through a “standard” terrestrial telecoms network(PSTN, IP, etc.) via the GES connection station can also be envisaged.

In this case:

-   -   the first user terminal REC1 transmits a first signal S1 to the        first low-orbit satellite LEO1,    -   the low-orbit satellite LEO1 receives and amplifies signal S1        transmitted by user terminal REC1 on the ground and transmits it        in the form of signal S2 to the geostationary satellite GEO1,    -   the geostationary-orbit satellite GEO1 receives signal S2, and        possibly signal S1, and re-transmits it in the form of signal S5        to the GES connection station.    -   the GES connection station receives signal S5 (combining, when        necessary, signals S1 and S2 contained in S5 by means of a Rake        receiver) and transmits it in the form of signal S6 to user        terminal REC3 on the ground via a standard terrestrial network.

It is noted that, in FIG. 1, the direct links between user terminalsREC1, REC2 and REC3 and geostationary satellite GEO1 are not shown inorder to simplify the figure.

Different approaches can be envisaged for the space repeater installedon-board a low-orbit satellite LEO1. LEO2, LEO3:

-   -   Either, preferably, a simple amplification without frequency        translation of the signal received from the GEO geostationary        satellite. However, this implies the use of an air interface        able to support the arrival of two signals comprising some        Doppler and delay differences. This is, for example, the case of        a CDMA—Code Division Multiple Access—type of air interface        associated to a Rake receiver.    -   Or, alternatively, the use of two separate signals (one for the        GEO satellite and one for the LEO satellite). For example, it is        possible to use a TDMA (Time Division Multiple Access) type of        air interface, known per se, envisaging either time-division        multiplexing to divide the capacity between the LEO and GEO        satellites (with guard intervals and Doppler pre-compensation at        the LEO traveling satellite), or the use of two sub-channels        (one for the GEO satellite and one for the LEO satellite).

In the implementation described here as an example, the first approachhas been selected because it offers a simple and effective solution.

In effect it takes advantage of the satellites' diversity since thesignals from both LEO and GEO satellites can be combined in a Rakereceiver to obtain a better signal-to-noise ratio. This technique ofimproving the signal-to-noise ratio makes it possible to obtain a lowerbit error rate, a lower EIRP (Effective Isotropically Radiated Power)transmitted power, or a greater margin in the link budget.

In addition, for a user terminal REC1, with a line of sight at the sametime to a low-orbit satellite LEO1 and to a geostationary satelliteGEO1, if the propagation conditions result in the loss of a link to oneof the satellites to which it is connected (due to the change in thegeometry of the link with the LEO varying as a function of time, orbecause of obstacles in the line of sight of one of the two satellitesLEO1 and GEO1), the other link can allow communication to be maintained.

This concept, of simple amplification without frequency translation ofthe signal received from the GEO satellite, can be implemented thanks tothe possibility offered by the Rake receiver, included in user terminalREC1, REC2, of combining different signals from different paths comingfrom an LEO low-orbit satellite and a GEO geostationary-orbit satellite.

In the scenario of satellite data communications, which is the subjectof this implementation, the multi-path component is generallynegligible. In this case, the Rake receiver is simply used to combineseveral direct signals coming from several LEO and GEO satellites, sincethe different signals can be considered fictitious “multiple path”components.

The signals received can then be combined in the user terminal REC1,REC2 according to three main algorithms, known to the person skilled inthe art and not therefore described any further here: by selecting thebest signal (known as “selection combining”), by simple equalcombination of the signals (known as “equal gain combining”), orweighted recombination of the signals to maximize the totalsignal-to-noise ratio (known as “maximal ratio combining”). The lastalgorithm (Maximum Ratio Combining) is the preferred algorithm becauseit is the most efficient in terms of the signal-to-noise ratio obtained.

One of the key issues related to combining signals is that each pathfollowed potentially has a very different length because of the relativeposition of the elements: user—LEO satellite—GEO satellite. In order tobalance the difference in propagation time, which also varies over time,suitable data buffers must be provided for at the Rake receiver. Thesizing of these buffers depends on the worst-case delay differencebetween the different paths, and on the maximum data transfer throughputused.

In the system proposed, the time difference remains less than 5 ms forthe constellation of traveling satellites LEO1, LEO2, LEO3 envisaged.The table in FIG. 3 gives several orders of magnitude of delays fordifferent LEO orbit altitudes relative to a GEO satellite.

It must also be noted that the communications services envisaged for thesystem according to the Invention are low-throughput data transmissions.Because of this, the necessary size of the data buffer remainsreasonable.

With this approach using a Rake receiver, in the case of a CDMAmultiplexing technique, the LEO and GEO satellites share the samefrequency band (MSS L band in this example) without generating harmfulinterference.

Frequency planning and issues of coverage must also be taken intoconsideration, because the LEO and GEO coverage areas must becoordinated in order to ensure the correct operation of the system. Inthe proposed approach the coverage of the GEO satellites consists of aglobal beam covering the whole of the visible surface of the Earth. Thisapproach makes it possible to avoid or limit the transfer procedures forthe LEO satellites (known to the person skilled in the art under theterm “hand over”) between different beams coming from one (or more) GEOsatellites. The LEO coverage is therefore included within the GEO'scoverage, as shown in FIG. 4. The LEO satellites thus simply relay thesignals of the GEO satellites under which they are located.

In the above example:

-   -   Satellite LEO1 relays the signals from and to satellite GEO1.    -   Satellites LEO2 and LEO3 relay the signals from and to satellite        GEO2.    -   There is no LEO satellite in satellite GEO3's coverage at the        point in time shown in FIG. 4. In effect, at that time satellite        LEO3 is connected to satellite GEO2.

As any one of the LEO satellites moves in the GEO satellites' coveragearea, it can have visibility with different GEO satellites. However, itis assumed that at a given time it is connected to a singlegeostationary satellite. When several GEO satellites are in thetraveling LEO satellites' area of visibility, different strategies canbe adopted for selecting the GEO satellite to which the LEO must beconnected (e.g. on a criterion of the best signal received at the LEOsatellite, or a geometric criterion of minimizing the distance betweenthe LEO and GEO which can be predicted in advance based on the ephemerisof the satellites). In the above example, the LEO satellite is connectedto the GEO satellite providing the best received signal.

With these hypotheses it is not necessary to devise complex frequencyplanning strategies and all the satellites (the three GEO satellites andthe three LEO satellites) can, for example, operate on a single CDMAchannel.

Unlike the approach of the prior state of the art, of the Orbcomm orArgos type, the system proposed is able to provide two-way datacommunications, based on the fact that the GEO satellite relays the LEOsatellites' communications.

According this approach, once the user terminal REC1 REC2 is in thecoverage area of a traveling satellite LEO1, LEO2, LEO3, it is possibleto communicate bi-directionally and in real-time with it. Therequirement for simultaneous visibility by the traveling satellite LEO1,LEO2, LEO3, the user terminal REC1, REC2 and a ground connection stationis eliminated, which consequently means that complete coverage of theEarth can be envisaged.

The delay for communicating with a user terminal REC1 on the ground isthus solely a function of the frequency of passage of the orbitingsatellites 1101. LEO2, LEO3, which depends directly on the orbit chosenfor these satellites and the number of these satellites (up to acontinuous coverage of the Earth).

BENEFITS OF THE INVENTION

Thanks to the combination of a constellation of traveling LEO satellites(which allows a more efficient service to be provided for the polarregions) and a GEO constellation (which provides a high-quality serviceto equatorial regions and low latitudes), the average duration of asatellites non-visibility for a user terminal REC1, REC2 is canceled orsignificantly reduced compared to the systems of the prior state of theart, especially when high angles of elevation are sought (in the case ofsatellite mobile communications the signal's blocking factor is reducedat high elevations, which leads to better service availability).

It is understood that a system as described thus provides significantlyimproved availability for users who have to operate in remote areaslittle covered by traditional communications systems.

Other advantages of the solution chosen are especially apparent when itis compared to the existing solutions using low-orbit satellites,geostationary satellites or hybrid constellations.

1/ The invention provides many benefits compared to a satellitecommunications solution employing a constellation of low-orbitsatellites (such as Orbcomm or Argos, for example).

Connection stations do not need to be deployed for communicationsbetween the LEO low-orbit satellites and the terrestrial networkinfrastructure. In effect, the connection station of the GEO satelliteor satellites ensures a permanent access to the LEO low-orbitsatellites.

It is not necessary to deploy Tracking/Telemetry and Command (TT&C)stations in the constellation of low-orbit satellites; the TT&C signalsare also relayed by the GEO satellite or satellites. It is understoodthat the low-orbit satellites are controlled, from the ground, by thecontrol stations of the stationary satellites, via these satellitesGEO1, GEO2, GEO3.

The design of the low-orbit satellites is significantly simplified(simplified relay function placed in orbit).

2/ The invention provides the following benefits compared to a satellitecommunications solution employing a geostationary satellite:

The geostationary satellite's coverage is extended to cover, forexample, the polar areas.

The link budget performance is improved; this allows, for example,miniaturization of user terminals, reduction in the power used by theterminals (less power to close the link budget, therefore greaterbattery autonomy for battery-operated terminals) or improved throughputsand availability.

A reduced latency time for access to the network and reception of theacknowledgment of receipt.

3/ The invention provides the following benefits compared to a hybridsatellite communications system (including the state of the art)comprising one or more geostationary satellites and a constellation oflow-orbit satellites:

A common protocol is used between the two systems, more efficient thaneach protocol taken separately.

The frequency band used is shared, with mechanisms ensuring thatinfra-system interference is limited.

As a result of its design the concept is especially suited to userterminals with low directivity and does not require pointing to bemaintained in the direction of the low-orbit satellites. In effect, itis sufficient for the user terminal to point towards a GEO or LEOsatellite to ensure communication.

The applications envisaged relate to the improvement of future mobilegeostationary satellite systems (MSS—Mobile Satellite Services),including aeronautical mobile satellite services such as AMSS(Aeronautical Mobile Satellite Service) and AMSRS (Aeronautical MobileSatellite Route Services), in the UHF. L, S, C or X band based on thedeployment of a much less complex low-orbit constellation than theexisting MSS constellations (such as Globalstar or Iridium) for voice ordata communications or exchanges of machine-to-machine (M2M) messages.

This concept can also be applied to data, television or radio mobilesatellite broadcasting systems (e.g. radio broadcasting in S-DABstandard using a BSS allocation in the L band or mobile televisionbroadcasting in DVB-SH standard using an MSS allocation in the S band).

Another use of this concept regards data exchanges for navigationapplications (in particular, maritime or aeronautical). In theseapplications, known to the person skilled in the art as SBAS (“SatelliteBased Augmentation System”), there are currently no means of coveringmobile devices located near the poles. The invention allows this problemto be remedied with a geostationary-orbit satellite and a low-orbittraveling satellite with a line of sight to the mobile device.

Similarly, any coverage of a shadow area of conventional communicationsdevices is potentially possible, once this area can come into the lineof sight of a low-orbit satellite or a traveling means.

Use of a telecommunications system or method according to the inventionthus makes it possible to extend the coverage area of communicationsmeans comprising shadow areas, and notably to extend the coverage areaof an SBAS system using a geostationary satellite.

In the case where the space repeater transmits in an adjacent channel tothat of the GEO satellite in a regenerative way, an advantage of theinvention concerns the possibility of a potential simplification of theexchange protocols between the user terminals and the LEO satellites. Inparticular, the LEO satellites can perform a conversion to a specificexchange protocol for the GEO (in order to take into account, forexample, propagation delay constraints specific to the GEO), oraggregate messages and optimize use of the bandwidth.

Another great appeal of this concept is the possibility of having apermanent quasi-real-time link between the mission and control networkand the LEO satellite constellation through the GEO relay and connectionstation.

It is further understood that the system does not necessarily requirethe deployment of a dedicated LEO or GEO satellite constellation. Indeedit is possible to use the available transmission capacities onalready-existing GEO satellite constellations. In this case, thefrequency band of the GEO constellation used is naturally chosen as theLEO satellites' working frequency band. This allows the problem to besolved of few frequency bands available for mobile satellite servicesusing non-geostationary satellites, and this therefore offers aregulatory advantage for the deployment of an LEO satelliteconstellation operating on a secondary basis in the same frequency bandas the GEO satellite or satellites.

Similarly, the functions envisaged for the LEO satellites can in fact becarried out by payloads installed as passengers on LEO satellites mainlydedicated other functions. In this case, the decisive criterion is theorbit envisaged for the LEO satellite. An advantageous choice is that ofEarth observation satellites, which frequently use a highly inclinedheliosynchronous orbit and thus cover the high latitudes. Thisutilization of payloads as passengers is naturally very advantageous interms of the cost of deploying the system.

The system described here therefore forms a simple and economic solutioncompared to other possible alternatives, such as:

1/ deploying a large number of ground stations so as to offer apermanent connection between the LEO satellites and the ground, which isan expensive and complex solution to implement, in particular forcovering the oceans (the Globalstar constellation is a good illustrationof this difficulty);

2/ using inter-satellite links so as to offer a permanent connectionbetween the LEO satellites and a limited number of ground stations. Thatsolution has the inconvenience of adding complexity and an additionalcost to the space segment (the Iridium constellation is a goodillustration of this solution).

VARIANTS OF THE INVENTION

Using the space diversity (or MIMO techniques) at the user terminal forrecombining the signals coming from both the GEO satellite and the LEOsatellite can be envisaged in order to further improve the link budget.

The satellite repeater can be a simple “transparent” analog repeater,which is the simplest solution but imposes design constraints on the airinterface so as to limit interference at the terminal between signalsfrom the GEO satellite and signals relayed by the LEO satellite.

An alternative solution consists of relaying the signal (in atransparent or regenerative way) in a sub-channel of a single band onboard the relay satellite. This solution requires a coordination entityfor coordinating the frequency plans between the GEO and LEO satellites.

The relay satellite constellation can also implement additionalfunctions (store & forward, aggregating signals).

The repeater satellite constellation can offer a global or partialcoverage of the Earth, depending on objectives.

The repeater satellite constellation can offer coverage that iscontinuous in time (for real-time services available at all times) orsolely an access with a certain delay (for non-real-time services) usingconstellations with a smaller number of satellites.

It is also clear that the concept described, using the same frequencyband for communications between the surface terminals and the repetitionmeans and for communications between the repetition means and thestationary means, can be applied solely on the forward channel or returnchannel, or in both directions.

1-14. (canceled)
 15. Satellite telecommunications system, comprising: atleast one transmitting/receiving surface terminal associated with a usersubstantially on a surface of the Earth; at least one geostationarysatellite configured to receive signals from and to transmit signals toa predefined coverage area with a line of sight to said at leastgeostationary satellite, and at least one traveling satellite movingabove the surface of the Earth and configured to perform at least one ofthe following using a same frequency band: repeat signals received fromsaid at least one transmitting/receiving surface terminal towards saidat least one geostationary satellite, or repeat signals received fromsaid at least one geostationary satellite towards said at least onetransmitting/receiving surface terminal, the same frequency band is usedto communicate between said at least one transmitting/receiving surfaceterminal and said at least one traveling satellite and between said atleast one traveling satellite and said least one geostationarysatellite; and wherein tracking/telemetry and command signals of said atleast one traveling satellite are relayed by said at least onegeostationary satellite.
 16. Satellite telecommunications systemaccording to claim 15, further comprising at least one ground connectionstation of said at least one geostationary satellite; and whereincommunications between said at least one traveling satellite and aterrestrial operator are performed via said at least one geostationarysatellite and said at least one ground connection station.
 17. Satellitetelecommunications system according to claim 16, wherein allcommunications between said at least one traveling satellite and theterrestrial operator are performed via geostationary satellites. 18.Satellite telecommunications system according to claim 15, wherein saidat least one travelling satellite moves in a low orbit.
 19. Satellitetelecommunications system according to claim 15, wherein said at leastone travelling satellite moves in a polar orbit.
 20. Satellitetelecommunications system according to claim 15, wherein the frequencyband is a L band.
 21. Satellite telecommunications system according toclaim 15, wherein the frequency band is used for direct communicationsbetween said at least one transmitting/receiving surface terminal andsaid at least one geostationary satellite.
 22. Satellitetelecommunications system according to claim 21, wherein said at leastone geostationary satellite is configured to transmit a signal S5 to aground connection station, the signal S5 comprises: a signal S1 receiveddirectly from said at least one surface terminal, and a signal S2received from said at least one travelling satellite, the signal S2corresponds to the signal S1 received and repeated by said at least onetravelling satellite.
 23. Satellite telecommunications system accordingto claim 15, wherein said at least one transmitting/receiving surfaceterminal utilizes a CDMA type interface and comprises a component tomanage arrival of two signals bearing Doppler and delay differences. 24.Satellite telecommunications system according to claim 23, wherein thecomponent to manage arrival of two signals bearing Doppler and delaydifferences is a Rake type of receiver.